1. Field of the Invention
The present invention relates to a low dropout regulator, and more particularly, to a quick-start low dropout regulator.
2. Description of the Prior Art
A conventional regulator is primarily used for generating a stable output voltage, which serves a stable voltage source of active elements or passive elements. However, while there are noises inside the regulator, preciseness of the regulator is significantly reduced. The noises are primarily brought by an input voltage source or elements within the regulator, where the noises from the elements within include flicker noises and thermal noises. For immunizing from the noises, a low dropout regulator is conventionally used for providing a stable voltage source. Since there is a significantly small voltage difference between the input voltage and the output voltage of the low dropout regulator, most of the above-mentioned noises are neutralized within the low dropout regulator.
Please refer to FIG. 1, which is a diagram of a conventional low dropout regulator 100. As shown in FIG. 1, the low dropout regulator 100 includes an input voltage source VIN, an output voltage source VOUT, a first error amplifier 110, a transistor 120, a reference voltage source 130, a second error amplifier 140, a first reference amplifier 150, a second reference amplifier 160, a low-pass filtering resistor 170, and a low-pass filtering capacitor 180. The low dropout regulator 100 primarily takes use of a low-pass filtering circuit, which is formed by the low-pass filtering resistor 170 and the low-pass filtering capacitor 180, to filter off the flicker noises within. However, since the low-pass filtering circuit is capable of filtering off most noises of the low dropout regulator 100, an operational rate of the low dropout regulator 100 is significantly affected by the low-pass filtering circuit; It is because that the low-pass filtering circuit produces a certain amount of delays itself, and a reaction rate of the low-pass filtering circuit is reduced by a charging/discharging time of the low-pas filtering capacitor 180.
Please refer to FIG. 2, which is a conventional low dropout regulator 200 acquiring a high operational rate. As shown in FIG. 2, the low dropout regulator 200 includes an error amplifier 210, transistors M7 and Q1, resistors R2, R3, and R10, a capacitor C6, and current sources 14 and 19. The low dropout regulator 200 has an input node at a node Vbgi shown in FIG. 2, and has an output node at a node Vbgo shown in FIG. 2 as well. A primary purpose of the low dropout regulator 200 lies in adjusting a voltage level at the node Vbgo to be close to a voltage level at the voltage Vbgi, under a condition that the voltage level at the node Vbgi is variable in correspondence to variations of an input voltage source. A voltage level at a node Vd, which is located at an intersection between resistors R2 and R3, is adjusted to a lower voltage level than the voltage level at the node Vbgi by voltage dividing performed by the resistors R2 and R3, i.e., a resistance of the resistor R2 is required to be much smaller than a resistance of the resistor R3.
Operations of the low dropout regulator 200 are described as follows. While the voltage level at the node Vbgi is high by following a corresponding voltage source having a high voltage level, and is assumed to be 0.74 volts, the voltage level at the node Vd becomes an intermediate voltage level, which is slightly lower than the voltage of the high voltage level at the node Vbgi and is assumed to be 0.7 volts. At this time, a voltage level at the positive input terminal of the error amplifier 210 is close to 0 volts, and a voltage level at the negative input terminal of the error amplifier 210 equals the intermediate voltage level, so that the transistor M7 implemented with a P-type MOSFET is switched on, and so that the voltage level at the node Vbgo may be gradually raised to the intermediate voltage level or slightly even higher with the aid of both the current source 19 and the switched-on transistor M7, for example, 0.7 volts or 0.72volts. Then, since the voltage level at the node Vbgo is higher than the voltage level at the node Vd, the error amplifier 210 outputs an output signal having a high voltage level so as to switch off the transistor M7, and so as to have the voltage level at the node Vbgo to be raised to close to the voltage level at the Vbgi with the aid of the resistor R10, instead of the current source 19 and the switched-on transistor M7. For example, the voltage level at the node Vbgo may be raised from 0.7 or 0.72 volts to 0.74 volts with the aid of the resistor R10.
During the operations of the low dropout regulator 200 shown in FIG. 2, the voltage level at the node Vbgo may be rapidly raised to be close to the voltage level at the node Vd with the aid of the current source 19, and may be restricted from being raised higher than the voltage level at the node Vbgi with the aid of the error amplifier 210. However, since the low dropout regulator 200 may adjust the voltage level at its output node to be close to the voltage level at its input node faster than the regulator 100 shown in FIG. 1, the low dropout regulator 200 also takes more elements and feedback structures so that larger circuit area or chip area is also required as a price in fabricating the low dropout regulator 200.